Fix behavior of Pages in @bibliography blocks

[goerz]

The Pages field now behaves exactly as Pages in @contents and @index blocks: all files are relative to the directory containing the current file.

Also, by popular demand, @__FILE__ is supported to refer to the current file (even though @contents and @index does not support this feature.

Closes #22

[goerz]

This is mostly internal changes, up to a few new sentences in the documentation that clarify the Pages field.

[goerz]

@kellertuer Can you have a look?

[kellertuer]

I can not evaluate the implementation itself, but the design and style looks good to me.

You write a bit skeptical here in the PR about the @__FILE__ not being used in the other blocks. Well, the other ones also do not read their content from Markdown files so a self-reference is not happening there. :)

[goerz]

I got confused about @autodocs. I thought it was also taking .md files in its Pages, but it doesn't, so @__FILE__ doesn't make sense there.

The @content and @index blocks do take .md files, though, no? Those seem like they have the same semantics for Pages as @bibliography, so I think JuliaDocs/Documenter.jl#2301 and JuliaDocs/Documenter.jl#2302 still apply, for consistency.

[goerz]

If there are no further comments, I will merge this sometime this weekend